The work in this project is designed to investigate the spatial organization among motoneuronal somata and their dendrites, and the afferent termination patterns projecting into their space. The medial gastrocnemius muscle will be injected with horseradish peroxidase (HRP) in order to produce retrogradely transported labeling of all the medial gastrocnemius motoneurons in cats. These same preparations will then be used in acute experiments wherein microelectrodes filled with either procion yellow or HRP will be used to select identified alpha motoneurons from the medial gastrocnemius motor nucleus. These neurons will then be intracellularly filled with the marker in order to define their morphology in three-dimensional space. The organization among dendrites of a number of these functionally related, selectively stained motoneurons will then be determined and related to the spatial organization of the whole motor nucleus in the same preparation. Studies in which one "type" of motoneuron (e.g., one controlling a slow twitch muscle unit) is filled with one marker and another "type" (e.g., one controlling a slow twitch muscle unit) is filled with another marker will also be carried out. Once the plan of organization among dendrites is defined, the projection of homonymous group Ia spindle afferents into that dendritic field will be studied by intracellularly injecting their axons at the dorsal root entry zone. Some of the general anatomical principles underlying sensory-motor organization should be revealed and may be applicable to the input-output morphology of other neuronal systems.